1. Field of the Invention
This invention relates generally to analog to digital converters and more particularly to an electron beam analog to digital converter which incorporates ultra-high speed sampling of the input signal by electron beam deflection.
2. Description of Related Art
Electron beam analog to digital conversion (EBADC) typically involves a cathode ray tube or oscilloscope which operates by deflecting electron beams proportional to the amplitude of the signal being converted. In a conventional implementation of such an analog to digital converter, the deflection angle of an electron beam is measured with a linear array of detector elements which is then converted from a thermometer-like code into a conventional binary format. Such apparatus is shown, for example in FIG. 1.
Referring now to FIG. 1, shown thereat is a field emission cathode 10, field electrode 12, electron optics 14, deflection plates 16, a linear detector array and a binary encoder 20 The detector array 18 additionally includes a plurality of detector elements 221 . . . 22n consisting of metal and/or semiconductor detector elements which are coupled to secondary gain or logic amplifiers which may be, for example, inverter type buffer amplifiers 241 . . . 24n having a predetermined gain. An input signal Vsignal, when applied to the deflection plates 16, causes a deflection of a focused electron beam 26 to strike at least one, but typically two or more of the detector elements 22. This constitutes a measure of the amplitude of the signal Vsignal. The detector element(s) 22 amplify the beam current, if necessary, by an amount sufficient to drive secondary gain or logic amplifiers 241 . . . 24n but in any case the detector array 18 produces an output which is fed to the binary encoder 20 and which in turn outputs an N bit binary signal corresponding to the amplitude of the input signal Vsignal.
The advantage of such an approach is that the deflection process is inherently linear and possesses extremely high bandwidth. Moreover, if the size can be reduced such that the deflection plates are as little as a few microns long even at modest beam energies of, for example, 30 v, the bandwidth can be as high as 300 GHz.
Accordingly, it is an object of the present invention to provide an improvement in analog to digital signal converters.
It is a further object of the invention to provide a high resolution, high speed analog to digital converter including ultra-high speed sampling of the analog input signal.
It is yet another object of the invention to provide an electron beam deflection analog to digital converter having extremely low timing jitter, high linearity, low power and radiation hardness.
These and other objects are achieved by a method and apparatus wherein a signal to be quantized is applied to the deflection circuitry of a cathode ray generation device where the electron beam is made to sweep an angle proportional to the signal amplitude in a first orthogonal direction across a linear array of detector elements. A periodic reference signal, preferably a sinusoidal signal, is applied to a second set of deflection plates which act to direct the electron beam in a second orthogonal direction causing the electron beam to sweep across the detector array where it generates a time sample of the input voltage at the crossovers. The detection voltage is then converted to an output signal having a binary value corresponding to the analog input value.
One aspect of the invention is directed to an electron beam analog to digital converter, comprising: an electron beam former generating an electron beam; an array of electron beam detector elements located in the path of the electron beam; first electron beam deflecting apparatus responsive to an analog input signal for deflecting the beam in a horizontal direction across the detector elements to a location proportional to the amplitude of the input signal, the electron beam exciting at least one of said detector elements which thereby generates an output signal indicative of the deflected beam angle; second electron beam deflecting apparatus responsive to a periodic reference signal for simultaneously deflecting the beam in a vertical direction across the detector elements causing a time sample of the detected input signal to be generated of the input signal during a crossover time interval; and a binary encoder coupled to the output signal and generating a binary output signal corresponding to the time sample of the input signal.
Another aspect of the invention is directed to a method of converting an analog signal to a digital signal, comprising the steps of: generating an electron beam; locating an array of electron beam detector elements in the path of the electron beam; deflecting the beam in a first direction across the detector elements to a location proportional to the amplitude of the input signal; simultaneously deflecting the beam in a second direction across the detector elements in response to a periodic reference signal; exciting at least one of said detector elements and an output signal indicative of the deflected beam angle; generating an output signal corresponding to a time sample of the input signal during a crossover time interval of the electron beam when deflected across the detector elements; and generating a binary output signal corresponding to the time sample of the input signal.
Further scope of applicability of the present invention will become apparent from the detailed description which is provided hereinafter. It should be understood, however, that the detailed description and specific examples, while disclosing the preferred embodiment of the invention, are provided by way of illustration only, since various changes and modifications and alterations coming within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.